VOL. LXVI.] PHILOSOPHICAL TRANSACTIONS. 40 



knowledge of this be of use to the salt makers, especially in cold countries ? 

 The sun is strong enough of itself between the tropics to dry away the sea- 

 water into salt ; and at the salt works near Lymington, they increase the saltness 

 of the seawater by drying it away in the sun before they boil it into salt. And this 

 seems to be another means of parting fresh water from the salt, which would 

 save expence in boiling it away, and may be of use in the cold countries, 

 and in winter. 



XXL An Account of the Meteorological Instruments used at the Royal Society's 

 House. By the Hon. Henry Cavendish, F. R. S. p. 373. 



Of the Thermometers, with reflections concerning some precautions necessary 

 to be used in making experiments with those instruments, and in adjusting their 

 Jix^d points. — The thermometers are both adjusted to Fahrenheit's scale: that 

 without doors is placed out of a two-pair-of-stairs window, looking to the north, 

 and stands about 2 or 3 inches from the wall, that it may be the more 

 exposed to the air, and the less atFected by the heat and cold of the house. 

 The situation is tolerably airy, as neither the buildings opposite to it, 

 nor those on each side, are elevated above it in an angle of more than 12°; but 

 as the opposite building is only 25 feet distant, perhaps the heat may be a little 

 increased at the time of the afternoon observation by the reflection from it. 

 In the middle of summer the sun shines on the wall of the house, against 

 which the thermometer is fixed, for an hour or two before the morning observa- 

 tion, but never shines on the thermometer itself, or that part of the wall close 

 to it, except in the afternoon, long after the time of observing. On the whole, 

 the situation is not altogether such as could be wished, but is the best the house 

 afforded. The thermometer within doors is intended chiefly for correcting the 

 heights of the barometer, and is therefore placed close to it. The room in 

 which it is kept looks to the north, and has sometimes a fire in it, but not 

 often. 



It has been too common a custom, both in making experiments with ther- 

 mometers and in adjusting their fixed points, to pay no regard to the heat of 

 that part of the quicksilver which is contained in the tube, though this is a 

 circumstance which ought by no means to be disregarded ; for a thermometer, 

 dipped into a liquor of the heat of boiling water, will stand at least 2° higher, 

 if it is immersed to such a depth that the quicksilver in the tube is heated to the 

 same degree as that in the ball, then if it is immersed no lower than the freezing 

 point, and the rest of the tube is not much warmer than the air. The only 

 accurate method is, to take care that all parts of the quicksilver should be heated 

 equally. For this reason, in trying the heat of liquor much hotter or colder 

 than the air, the thermometer ougJit, if possible, to be immersed almost as far as to 



VOL. XIV. H 



